ABSTRACT Entry into S phase is a key event for regulation of mammalian cell proliferation. The controlled activation of histone gene expression at the G1/S phase transition is essential for chromatin packaging of nascent DNA. Our program has identified HiNF-P as the principal transcriptional regulator of histone genes, the end-point molecule for the cyclin E/ CDK2/ p220NPAT pathway, and HiNF-P deficiency impairs cell cycle progression through S phase. The discovery and characterization of HiNF-P represents one of the most important accomplishments of our program and the proposed studies will build on this major result to open a new dimension to cell cycle control. Our central hypothesis is that the cyclin E/ CDK2/ p220NPAT/ HiNF-P gene regulatory cascade is a principal cell cycle pathway that functions to achieve competency for histone H4 biosynthesis to support packaging of DNA as chromatin. To address this hypothesis, we will combine biochemical, molecular, cellular and in vivo genetic strategies to define how the HiNF-P/p220NPAT pathway supports S phase activation and progression. First, we will characterize protein-protein interaction domains and post-translational modifications that control the activity of the HiNF-P/p220NPAT complex, as well as the in situ integration of regulatory signals at subnuclear sites ('Histone Locus Bodies') that mediate histone gene transcription and mRNA processing (Specific Aim 1). We will then examine how HiNF-P deficiency affects cell cycle progression in normal and tumor cells and address HiNF-P related mechanisms involved in regulating cell proliferation (Specific Aim 2). To establish the in vivo relevance of the HiNF-P pathway, we will investigate whether HiNF-P is important for normal development in vivo during embryogenesis and post-natal growth (Specific Aim 3). The three proposed aims are designed to provide an integrated understanding of the molecular, cellular, and biological contributions of the cyclin E/CDK2/p220NPAT/HiNF-P pathway to cell proliferation in normal and tumor cells.